Respiratory syncytial virus (RSV) is an enveloped non-segmented negative-strand RNA virus in the family Paramyxoviridae, genus Pneumovirus. It is the most common cause of bronchiolitis and pneumonia among children in their first year of life. RSV also causes repeated infections including severe lower respiratory tract disease, which may occur at any age, especially among those with compromised cardiac, pulmonary, or immune systems.
To infect a host cell, paramyxoviruses such as RSV, like other enveloped viruses such as influenza virus and HIV, require fusion of the viral membrane with a host cell's membrane. For RSV the conserved fusion protein (RSV F) fuses the viral and cellular membranes by coupling irreversible protein refolding with juxtaposition of the membranes. In current models based on paramyxovirus studies, the RSV F protein initially folds into a metastable “pre-fusion” conformation. During cell entry, the pre-fusion conformation undergoes refolding and conformational changes to its stable “post-fusion” conformation.
The RSV F protein is translated from mRNA into an approximately 574 amino acid protein designated F0. Post-translational processing of F0 includes removal of an N-terminal signal peptide by a signal peptidase in the endoplasmic reticulum. F0 is also cleaved at two sites (approximately 109/110 and approximately 136/137) by cellular proteases (in particular furin) in the trans-Golgi. This cleavage results in the removal of a short intervening sequence and generates two subunits designated F1 (˜50 kDa; C-terminal; approximately residues 137-574) and F2 (˜20 kDa; N-terminal; approximately residues 1-109) that remain associated with each other. F1 contains a hydrophobic fusion peptide at its N-terminus and also two amphipathic heptad-repeat regions (HRA and HRB). HRA is near the fusion peptide and HRB is near the transmembrane domain. Three F1-F2 heterodimers are assembled as homotrimers of F1-F2 in the virion. A suitable vaccine for infants against RSV infection is not currently available, but is desired.
Previous attempts at RSV vaccines have led to failed clinical trials wherein the vaccine did not protect against infection and in fact was associated with increased risk of severe RSV disease when the vaccinated children became infected. Kim, H. W., et al., Am. J. Epidemiol., 89:422-434 (1969). Traditional immunization methods for vaccinating infants from RSV are not available.
Thus, there is a need for improved RSV immunization regimens.